This invention relates to a mechanism for limiting the torque applied by tools, and is more particularly concerned with such mechanism in hand tools, such as wrenches and the like.
In hand tools, and the like, having torque controlling mechanisms, it is generally desirable to permit the torque release setting to be changed by the user as necessary. Customarily, different torque value adjustments can be quickly and easily effected with the assistance of a generally micrometer incremented scale and thimble operable by means of a handle on the tool. If torque release values are compared with tester readings at all micrometer settings from lowest to highest, a characteristic curve of linear progression of more or less comparable quantities is presented. The system of these comparisons represents the "spring rate". If the series of release values do not match the progression of all other micrometer settings accurately enough, the spring rate must be changed. In the case of all common torque tools (micrometer style wrenches), the spring rate can only be changed if the pivot block is substituted for one with more appropriate mechanical properties, i.e. of proper height. Changing the pivot block requires complete disassembly of the tool. Due to inherent variables with the design, it may take several trials with different blocks to obtain proper accuracy through the full range of settings. Spring rate change results from use of pivot blocks having different length-to-width ratio. Some suppliers of the above tools may provide as many as 40 types of replacement pivot blocks to attempt proper spring rate.
Prior torque limiting or controlling mechanisms have had no provision for calibrating both directions of rotation independently. Torque control may be required for items in the clockwise or counterclockwise direction or both. While torque wrenches, for example, may be set to release according to a setting in either direction, the calibration choice is limited to one direction or the other. For example, in a typical mechanism accuracy may be only within a plus or minus 4% of the reading in the clockwise direction and plus or minus 6% of the reading in the counterclockwise direction from 20% of the full scale reading to the full scale reading. Differences in the torque release values between clockwise and counterclockwise applications at the same setting are caused by variations in machining, variations in friction from side to side between plunger and side of case, variations due to coil spring orientation, etc.
In prior torque control mechanisms, no provision is made to cancel out the nonproportional torque output compared to the progression of torque settings. Due to the nature of the release mechanism which includes the pivot block operation, a non-linearity exists between the progression of spring compression (as dictated by transverse screw thread pitch) and the actual release torque. At the highest settings, the system completely "breaks over", i.e. releases, as soon as the initial spring pressure is overcome. An audible signal results. At lowest settings, when the initial spring pressure is overcome, the pivot block begins to rotate, but spring pressure increases at a greater rate than the decrease in force caused by the increasing block angle. This results in higher output torque release values to be realized at low settings and complete loss of the audible signal. The higher release values at low settings can be compensated for by selecting a shorter pivot block.
Attempts at calibration show that a "droop" now exists in prior mechanisms in the characteristic curve of the torque setting and release comparisons. For example, after selection and installation of correct pivot block and proper low setting, a 100 lb. ft. capacity torque wrench may be checked for accuracy at various settings. At a setting of 20, the release takes place at 20. At the highest setting of 100, the release takes place at 100. However, at a setting of 30, the typical release value in prior tools has been only 29. This non-linearity is typical for the prior torque wrenches utilizing pivot blocks. A greater allowance for accuracy variation must be made in the prior devices because of this non-linear function.
Prior devices of the kind under discussion have inadequate provision for efficient low value setting and exact alignment of scale to thimble increments. Initial settings are accomplished with the use of multiple spacers and washers requiring at least partial disassembly of the tool. Sets of solid disk spacers are used to allow for main springs of different lengths and to establish the lowest limit of tool torque release setting. Sets of washers of several thicknesses are generally used to establish proper alignment between the thimble and scale increments of the tools.